The first step in the refining of crude petroleum (crude oil) is normally distillation to separate the complex mixture of hydrocarbons into fractions of differing volatility. Distillation requires heating to vaporize as much of the liquid as possible without exceeding an actual temperature of about 650° F., since higher temperatures lead to thermal decomposition. The fraction which is not distillable at 650° F. and atmospheric pressure is commonly further distilled under vacuum, such that an actual temperature of 650° F. can vaporize even more liquid, equivalent to a theoretical equivalent of 1050° F. at atmospheric pressure. The remaining undistillable liquid is referred to as petroleum residue, distillation residue, or simply “1050+resid.” This fraction is of low value as a fuel because of its high viscosity and low volatility. Sulfur is concentrated in the residua typically to about 2.5 times the concentration of sulfur in the crude oil. Currently, petroleum residua are typically subjected to destructive thermal decomposition to yield cracked liquid and gas, and solid petroleum coke. The reactors for thermal decomposition are called cokers, and they may be fluidized bed reactors or stationary drums. Coker liquids require much upgrading by reaction with hydrogen to be blended with other petroleum products. Other outlets for residua include blending with lower viscosity distillates to make residual fuel oil, or use as paving or roofing asphalts.
However, since the residue fraction typically constitutes more than 20% by mass of the starting crude oil, there is high incentive to convert it to a clean burning fuel such as methane which may be fully substituted for natural gas or added to natural gas as a supplement.
Some crude oils yield on distillation more than 50% by mass of residue. Such crude oils are referred to as heavy crude oils, and it may be advantageous to convert such oils directly to methane without distillation or to perform only atmospheric pressure distillation and convert the atmospheric distillation residue to methane.
In addition to crude oil distillation residue and heavy crude oils, some petroleum refining processes such as catalytic cracking and fluidized bed coking have distillation steps which yield high boiling fractions which are typically coked, but might have higher value if converted to methane. For purposes of the present specification and claims, the term petroleum residue will be used to mean any such feedstock containing more than 50% residue which does not vaporize below an atmospheric pressure equivalent temperature of 1050° F.
The closest prior art related to the present invention is disclosed in several now expired patents: U.S. Pat. No. 3,958,957 (Koh, et al, May 25, 1976) teaches equilibrium limited methane formation from hydrogen and carbon monoxide in the presence of carbon-alkali metal catalysts. U.S. Pat. No. 4,077,778 (Nahas, et al, Mar. 7, 1978), and U.S. Pat. No. 4,094,650 (Koh, et al, Jun. 13, 1978), teach the alkali-metal catalyzed conversion of coal by reaction with steam to form methane and carbon dioxide in a substantially thermally neutral reaction effected by recycling the endothermic reaction products, hydrogen and carbon monoxide, so as to prevent their net formation in the reactor. The preferred temperature and pressure ranges such that methane is the only stable hydrocarbon and is produced at reasonable rates and concentrations are discussed by Nahas in Fuel Vol. 62:239-241 (February 1983). The Fuel article also describes the role of the reaction kinetics of catalyzed carbon gasification and the importance of achieving high steam conversion.
The '778 and '650 patents disclose that the process chemistry is applicable to carbonaceous feeds in general, but their detailed descriptions teach conversion of coal, and do not enable one skilled in the art to practice the conversion of liquid feeds such as petroleum residua without undue experimentation to determine appropriate means of mixing feed and catalyst, or relative amounts of feed and catalyst.
Results of the research leading to the development of the catalytic coal gasification process were published by Kalina and Nahas in DOE Report FE-2369-24 (December 1978). As reported therein and subsequently by Euker and Reitz in DOE Report FE-2777-31 (November 1981), it was found that the most effective way to contact coal and catalyst was to mix dried coal with an aqueous solution of alkali metal (preferably potassium) carbonate or hydroxide and subsequently dry the mixture to leave the equivalent of 10-20% potassium carbonate on the coal. Since coal typically contains about 10% inorganic mineral matter, the inorganic portion must be purged from the reactor, taking with it some unconverted carbon and all of the added catalyst. Clay minerals in the coal reacted with potassium to form kaliophilite, a catalytically inactive potassium aluminosilicate. Potassium was recovered from the purged solids by a combination of water washing and lime-water digestion, but as much as a third of the original catalyst remained irreversibly in the purged solids. The recovery and recycle of spent catalyst was therefore expensive and only partially effective.
The teachings of the prior art were based on coal for which the hydrocarbon portion of the feedstock is generally accompanied by 20% to 30% by weight of inorganic matter consisting of naturally occurring mineral matter in the coal plus the added alkali metal compound as catalyst. The reactor volume, and thus the catalyst holdup, were based on the solids residence time required for substantially complete gasification of the carbon before solids were purged from the reactor to prevent buildup of inorganic coal mineral matter. Reactors were thus sized for solids retention time. The rates of feed, steam, and recycle gas were determined by material balance, but this approach is not useful for determining the appropriate contacting of the feed, steam, and recycle gas to a substantially captive bed of catalyst for conversion of petroleum residua or heavy oil.
In addition, it was found that in fixed-bed batch experiments, the raw product gas was in chemical equilibrium with respect to methane, hydrogen, carbon monoxide, carbon dioxide, and unreacted steam. Steam conversion was kinetically limited and the reaction rate was found to be inhibited by reaction products. However, it was recognized that commercial reactors would need to utilize fluidized beds instead of fixed bed reactors, because fluidization is necessary to facilitate temperature control of the adiabatic reaction, to accommodate reasonable gas velocities at low pressure drops, and facilitate the feeding and withdrawing of solids. Unlike in fixed beds, the turbulent mixing in fluidized beds exhibits gas backmixing, a phenomenon which allows product gas to recirculate within the reactor and thereby inhibit the reaction rate throughout the reactor. In fluidized bed pilot plant experiments, the product methane and carbon dioxide were generally found to be at lower than equilibrium concentrations with hydrogen, carbon monoxide, and steam. Consequently it was determined that a single stage fluidized bed reactor would require longer solids residence times and reactor holdup than would be needed without gas backmixing.
The referenced U.S. Pat. No. 4,077,778 teaches a two-stage process for more complete gasification of coal particles, in which fine particles and overflow particles from a first stage are conveyed to a second stage for further reaction. In the '778 patent however, the two stages are in parallel with respect to the flow of the gasification medium. As a result, this two-stage configuration does not address the gas backmixing which has been found to inhibit the reaction rate with reaction products.
The increased carbon conversion taught by the '778 patent mitigates the loss of carbon in fine particulates entrained from the main fluidized bed reactor, but there remains the problem that fine particles are continuously generated by attrition and gasification in both stages. There is no means for particle growth by coalescence or agglomeration to offset the effects of attrition and gasification, and as a result, particles escaping from the second stage carry some carbon which is lost from the system.